The present invention relates to methods for terminating both of the constituent encoders of a turbo code and developing puncturing patterns applicable at a trellis termination stage that ensures the same number of transmitted bits for each trellis stage during the information bit transmission and trellis termination stages.
The process of forward and reverse link turbo encoding and decoding, specifically for Code Division Multiple Access (CDMA) communications channels, is thoroughly described in copending U.S. patent application Ser. No. 09/248,338 of Eroz, et al., for SETS OF RATE-COMPATIBLE UNIVERSAL TURBO CODES NEARLY OPTIMIZED OVER VARIOUS RATES AND INTERLEAVER DELAYS, filed Feb. 11, 1999, and copending U.S. patent application Ser. No. 09/235,582 of Eroz, et al., for FORWARD ERROR CORRECTION SCHEME FOR DATA CHANNELS USING UNIVERSAL TURBO CODE, filed Jan. 22, 1999, both of which are incorporated herein by reference.
In a convolutional encoding scheme, tail bits are inserted after information bits, to zero out all shift registers of an encoder. For feed forward encoders, tail bits are equal to zero. For feedback encoders the value of tail bits depend on the contents of the shift register current values.
A turbo encoder consists of a parallel concatenation of two (2) or more recursive (feedback) convolutional encoders. Because each constituent encoder processes the information bits in a different order due to a turbo interleaver, it is not possible to terminate all constituent encoders by the same tail bits.
A trellis termination method general enough to be used for a set of turbo codes with different code rates as in the third generation CDMA systems is desirable. Included in the desirable general method is a method of puncturing tail bit sequences.